Fix TAP tests with "make installcheck".

I neglected that the prove_installcheck rule also needs to also define
PG_REGRESS, like prove_check does.

src/Makefile.global.in

@@ -334,7 +334,7 @@ endef
 ifeq ($(enable_tap_tests),yes)
 
 define prove_installcheck
-cd $(srcdir) && TESTDIR='$(CURDIR)' PATH="$(bindir):$$PATH" PGPORT='6$(DEF_PGPORT)' top_builddir='$(CURDIR)/$(top_builddir)' $(PROVE) $(PG_PROVE_FLAGS) $(PROVE_FLAGS) t/*.pl
+cd $(srcdir) && TESTDIR='$(CURDIR)' PATH="$(bindir):$$PATH" PGPORT='6$(DEF_PGPORT)' top_builddir='$(CURDIR)/$(top_builddir)' PG_REGRESS='$(CURDIR)/$(top_builddir)/src/test/regress/pg_regress' $(PROVE) $(PG_PROVE_FLAGS) $(PROVE_FLAGS) t/*.pl
 endef
 
 define prove_check